There are an IMPATT (impact ionization avalanche and transit time device) diode used and a BARITT (barrier inject transit time device) diode as high frequency devices which amplify high frequency waves into high power waves or generate an oscillation of high frequency waves with high power.
FIG. 1 shows a typical example of IMPATT diodes. An n.sup.+ -type (or p.sup.+ -type) semiconductor layer, a less doped semiconductor layer (.nu.), an n-type (or p-type) semiconductor layer and a p.sup.+ -type (or n.sup.+ -type) semiconductor layer make strata in this order in the diode. Electrodes are fitted on both ends (n.sup.+ -type and p.sup.+ -type of layers) of the strata. A reverse bias voltage is applied to the diode. Namely the electrode of the n.sup.+ -type layer is connected to the positive terminal of an electric power source, and the electrode of the P.sup.+ -type layer is connected to the negative terminal of the power source. This reverse bias voltage induces a carrier avalanche in the less doped semiconductor layer (.nu.) near the n-type layer. The electrons generated by the avalanche run through the less doped semiconductor layer (.nu.) to the n.sup.+ -type layer with saturated velocity. This phenomenon induces negative resistance in the diode. The occurrence of negative resistance enables the diode to generate microwave oscillation. The diode of FIG. 1 is a typical IMPATT diode which has a pn-junction. But there is another type of IMPATT diode in which the pn-junction is replaced by a Schottoky junction between a metal and a semiconductor.
A BARITT diode has a structure in which a metal layer, a p-type layer (or n-type layer) of semiconductors and a metal layer make strata in this order (FIG. 4). One junction between the metal and one semiconductor layer is a Schottoky junction. In the same way as the IMPATT diodes, when a reverse bias voltage is applied to the diode, majority carriers are injected to the semiconductor layer. The action of the majority carriers generates microwave oscillation. There is another kind of BARITT diode which has a pn-junction instead of the Schottoky junction between a metal and a semiconductor.
The semiconductor material of both high frequency devices is silicon or gallium arsenide. Other semiconductor material has not been used as the material of high frequency devices so far.
In the high frequency devices abovementioned, much heat is generated in the semiconductor region in the action of microwave oscillation. To keep the microwave oscillation stable, the heat incessantly generated in the action of oscillation should be effectively emitted out of the diode. Various kinds of cooling devices have been invented in order to eliminate the heat from the diodes. A typical method for cooling is fitting the high frequency device to a heatsink with large heat capacity. In the structure the heat quickly conducts in the heatsink and emanates from the surfaces of the heatsink into the air.
However silicon and gallium arsenide have poor heat conductivity. In the conventional high frequency devices made of silicon or gallium arsenide, the heat generated by the microwave oscillation slowly conducts in the semiconductor and does not attain to the heatsink so fast that the semiconductor region is kept cool enough. Due to the poor heat conductivity of silicon or gallium arsenide, if a large amount of electric current is applied to them, the conventional high frequency device will be broken down by overheating. To avoid the heat-induced breakdown, the input current shall be restricted to a low level. Because of the low input power, the output power of microwave oscillation is naturally low in the devices of silicon or gallium arsenide.
There is another problem regarding the heat dissipation. The junction between the device and the heatsink is accompanied with large heat resistance. Although the heatsink has high heat conductivity, the heat generated by microwave oscillation is not effectively dissipated because of the low conductivity of semiconductors Si or GaAs and the high heat resistance at the interface between the device and the heatsink. Thus fitting the heatsink to the device cannot solve the problem that poor heat dissipation restricts the output power of microwave oscillation to low level.
Besides poor heat conductivity, in the semiconductors, silicon or gallium arsenide, the output power and the maximum oscillation frequency of the devices are restricted by the insulation breakdown voltage and the carrier mobility which are inherently determined by the properties of the semiconductors.
Both silicon and gallium arsenide have a low insulation breakdown voltage. High reverse bias voltage cannot be applied to silicon or gallium arsenide devices because of the low insulation breakdown voltage. Thus the silicon or gallium arsenide devices cannot accomplish high output power of microwave oscillation, because the output power is in proportion to the input voltage.
The saturation mobilities of silicon and gallium arsenide are about 1.times.10.sup.7 cm/sec, which is not fast enough to generate high power oscillation of hundreds of gigaherz (GHz). Low saturation mobility is the most fatal drawback of silicon or gallium arsenide semiconductor device.
The purpose of this invention is to solve the difficulties and to provide a high power and high frequency device which amplifies high frequency signals or generates high frequency oscillation. Another purpose of the invention is to provide a high power and high frequency device which effectively works under high temperature environment.